Needed to add 3 chain links at 12g total – yields a net increase of 78g

Days Tested: 20

Locations Tested: Park City, UT; Moab, UT

Wolf Tooth writes: “The GC42 cassette cog is so big that when machining the first batch, the machinist thought he made a mistake!” They weren’t kidding. In a pinch I think this cog could serve as a decent plate; this thing is big.

Large add-on cogs for 10 speed cassettes give a 1×10 drivetrain a gear range closer to that of a 1×11 drivetrain, only at a fraction of the cost (assuming you already own a 1×10 drivetrain). But the question on many minds is: aside from this increased range, what drawbacks does running a large add-on cog on your 10 speed cassette present? I aimed to answer that in testing out the Wolf Tooth GC42 cassette cog.

Installation

If you have some experience installing cassettes and have either the Shimano M771 or M980 cassette compatible with the GC42, setting it up should be a breeze. Assuming you are using a 1x setup, you’ll need to shift your derailleur into the largest cog and then adjust your B-screw until the top pulley on the derailleur is as close to the cog as it can be without any contact or drag between the two. I found that I was able to adjust the stock screw on my derailleur to a point where I thought it would probably work, though I wasn’t sure; Wolf Tooth states that a longer screw is only needed on 50% of Shimano derailleurs. I didn’t have a longer screw on hand and was itching to ride, so I decided to give it a shot.

Also make sure to check your chain length to see if you’ll need to add links. If you do, make sure they match your chain for wear; if you put new links in an old chain or add old links to a new chain, everything will feel awful.

Tom Collier running the Wolf Tooth GC 42t cassette cog.

Shifting

Shifts up to the 42t cog were fine, but there was some hesitation on the way down and I could feel the derailleur cage catch the cog on the way down. This would have been ok if the new cog were used only rarely as a bailout gear, but shifts down to the next smallest cog also responded at only 70% the speed of regular shifts, and I was nervous about damage to the derailleur cage.

I caved and went to the hardware store to pick up a longer b-screw (M4 x 25), which allowed for a bit more adjustment. With the longer screw there was no longer any contact between the derailleur cage and the new 42t cog.

The Wolf Tooth GC 42t cog installed on a Shimano CS-M771 cassette.

With the longer screw installed, there was no appreciable drag, shifts up to the cog were just as smooth as any other gear shift, and shifts down to the next smallest cog were probably 90-95% as smooth and quick as other shifts.

(Wolf Tooth has also just released a part they call the GoatLink that should obviate the need for the longer screw.)

3 Comments

Nice in-depth review. I hear you on the range advantage of the 42 over the 40, but having tried both, I can say that the shifting with the 40 and the stock derailleur is noticeably better across the entire cassette, since the b-screw doesn’t have to be screwed in so much further than originally intended by Shimano.

At about 600 miles, I gave up, and went back to a stock XT cassette, since, as you predicted, wear became very significant.

Also, if you have to buy a new cassette to accomodate the WT 40 or 42 tooth, take that cost into consideration. That’s a 100 dollar cassette and a 65 dollar replacement to one of the cogs. Sure, I want a 42 tooth granny, but it’s not worth the SRAM prices and with Praxis’ coming out with a 40t for ~the price of an XT, that’s probably where I’ll land without upgrading to 1×11 (Gx or XT). First world problems.